Ds Roms Google Drive

[Doris Joo]

During the 1990s and early 2000s, CD-ROMs were popularly used to distribute software and data for computers and fifth generation video game consoles. DVDs as well as downloading started to replace CD-ROMs in these roles starting in the early 2000s, and the use of CD-ROMs for commercial software is now uncommon.

On early audio CD players that were released prior to the advent of the CD-ROM, the raw binary data of CD-ROM was played back as noise. To address this problem, the subcode channel Q has a "data" flag in areas of the disc that contain computer data rather than playable audio. The data flag instructs CD players to mute the audio.[14][15]

CD-ROMs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. The most common size of CD-ROM is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as shaped compact discs in numerous non-standard sizes and molds (e.g., business card-sized media), also exist.

Data is stored on the disc as a series of microscopic indentations called "pits", with the non-indented spaces between them called "lands". A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands. Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This is converted into binary data.

Several formats are used for data stored on compact discs, known as the Rainbow Books. The Yellow Book, created in 1983,[6][16] defines the specifications for CD-ROMs, standardized in 1988 as the ISO/IEC 10149[1] standard and in 1989 as the ECMA-130[17] standard. The CD-ROM standard builds on top of the original Red Book CD-DA standard for CD audio. Other standards, such as the White Book for Video CDs, further define formats based on the CD-ROM specifications. The Yellow Book itself is not freely available, but the standards with the corresponding content can be downloaded for free from ISO or ECMA.

There are several standards that define how to structure data files on a CD-ROM. ISO 9660 defines the standard file system for a CD-ROM. ISO 13490 is an improvement on this standard which adds support for non-sequential write-once and re-writeable discs such as CD-R and CD-RW, as well as multiple sessions. The ISO 13346 standard was designed to address most of the shortcomings of ISO 9660,[18] and a subset of it evolved into the UDF format, which was adopted for DVDs. A bootable CD specification, called El Torito, was issued in January 1995, to make a CD emulate a hard disk or floppy disk.

Pre-pressed CD-ROMs are mass-produced by a process of stamping where a glass master disc is created and used to make "stampers", which are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable (CD-R) and rewritable (CD-RW) discs are manufactured by a different method, whereby the data are recorded on them by a laser changing the properties of a dye or phase transition material in a process that is often referred to as "burning".

The structures used to group data on a CD-ROM are also derived from the Red Book. Like audio CDs (CD-DA), a CD-ROM sector contains 2,352 bytes of user data, composed of 98 frames, each consisting of 33 bytes (24 bytes for the user data, 8 bytes for error correction, and 1 byte for the subcode). Unlike audio CDs, the data stored in these sectors corresponds to any type of digital data, not audio samples encoded according to the audio CD specification. To structure, address and protect this data, the CD-ROM standard further defines two sector modes, Mode 1 and Mode 2, which describe two different layouts for the data inside a sector.[2] A track (a group of sectors) inside a CD-ROM only contains sectors in the same mode, but if multiple tracks are present in a CD-ROM, each track can have its sectors in a different mode from the rest of the tracks. They can also coexist with audio CD tracks, which is the case of mixed mode CDs.

Before being stored on a disc with the techniques described above, each CD-ROM sector is scrambled to prevent some problematic patterns from showing up.[17] These scrambled sectors then follow the same encoding process described in the Red Book in order to be finally stored on a CD.

CD-ROM XA is an extension of the Yellow Book standard for CD-ROMs that combines compressed audio, video and computer data, allowing all to be accessed simultaneously.[20] It was intended as a bridge between CD-ROM and CD-i (Green Book) and was published by Sony and Philips, and backed by Microsoft, in 1991,[21] first announced in September 1988.[22] "XA" stands for eXtended Architecture.

CD-ROM XA defines two new sector layouts, called Mode 2 Form 1 and Mode 2 Form 2 (which are different from the original Mode 2). XA Mode 2 Form 1 is similar to the Mode 1 structure described above, and can interleave with XA Mode 2 Form 2 sectors; it is used for data. XA Mode 2 Form 2 has 2,324 bytes of user data, and is similar to the standard Mode 2 but with error detection bytes added (though no error correction). It can interleave with XA Mode 2 Form 1 sectors, and it is used for audio/video data.[19] Video CDs, Super Video CDs, Photo CDs, Enhanced Music CDs and CD-i use these sector modes.[23]

CD-ROM capacities are normally expressed with binary prefixes, subtracting the space used for error correction data. The capacity of a CD-ROM depends on how close the outward data track is extended to the disc's outer rim.[25] A standard 120 mm, 700 MB CD-ROM can actually hold about 703 MB of data with error correction (or 847 MB total). In comparison, a single-layer DVD-ROM can hold 4.7 GB (4.7 109 bytes) of error-protected data, more than 6 CD-ROMs.

CD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, SATA, FireWire, or USB interface or a proprietary interface, such as the Panasonic CD interface, LMSI/Philips, Sony and Mitsumi standards. Virtually all modern CD-ROM drives can also play audio CDs (as well as Video CDs and other data standards) when used with the right software.

CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

CD-ROM drives are rated with a speed factor relative to music CDs. If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 Kbyte/s, commonly called "1" (with constant linear velocity, short "CLV"). At this data rate, the track moves along under the laser spot at about 1.2 m/s. To maintain this linear velocity as the optical head moves to different positions, the angular velocity is varied from about 500 rpm at the inner edge to 200 rpm at the outer edge. The 1 speed rating for CD-ROM (150 Kbyte/s) is different from the 1 speed rating for DVDs (1.32 MB/s).

When the speed at which the disc is spun is increased, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8 speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 Kbyte/s. Above 12 speed most drives read at Constant angular velocity (CAV, constant rpm) so that the motor is not made to change from one speed to another as the head seeks from place to place on the disc. In CAV mode the "" number denotes the transfer rate at the outer edge of the disc, where it is a maximum.20 was thought to be the maximum speed due to mechanical constraints until Samsung Electronics introduced the SCR-3230, a 32 CD-ROM drive which uses a ball bearing system to balance the spinning disc in the drive to reduce vibration and noise. As of 2004, the fastest transfer rate commonly available is about 52 or 10,400 rpm and 7.62 MB/s. Higher spin speeds are limited by the strength of the polycarbonate plastic of which the discs are made. At 52, the linear velocity of the outermost part of the disc is around 65 m/s. However, improvements can still be obtained using multiple laser pickups as demonstrated by the Kenwood TrueX 72 which uses seven laser beams and a rotation speed of approximately 10.

The first 12 drive was released in late 1996.[26] Above 12 speed, there are problems with vibration and heat. CAV drives give speeds up to 30 at the outer edge of the disc with the same rotational speed as a standard (constant linear velocity, CLV) 12, or 32 with a slight increase. However, due to the nature of CAV (linear speed at the inner edge is still only 12, increasing smoothly in-between) the actual throughput increase is less than 30/12; in fact, roughly 20 average for a completely full disc, and even less for a partially filled one.

Software distributors, and in particular distributors of computer games, often make use of various copy protection schemes to prevent software running from any media besides the original CD-ROMs. This differs somewhat from audio CD protection in that it is usually implemented in both the media and the software itself. The CD-ROM itself may contain "weak" sectors to make copying the disc more difficult, and additional data that may be difficult or impossible to copy to a CD-R or disc image, but which the software checks for each time it is run to ensure an original disc and not an unauthorized copy is present in the computer's CD-ROM drive.[citation needed]

Manufacturers of CD writers (CD-R or CD-RW) are encouraged by the music industry to ensure that every drive they produce has a unique identifier, which will be encoded by the drive on every disc that it records: the RID or Recorder Identification Code.[32] This is a counterpart to the Source Identification Code (SID), an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording plants.

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